U.S. patent application number 11/990067 was filed with the patent office on 2010-10-28 for particle-optical system.
This patent application is currently assigned to Carl Zeiss SMS GmbH. Invention is credited to Helmut Falkner, Elmar Platzgummer, Gerhard Stengl.
Application Number | 20100270474 11/990067 |
Document ID | / |
Family ID | 36061724 |
Filed Date | 2010-10-28 |
United States Patent
Application |
20100270474 |
Kind Code |
A1 |
Platzgummer; Elmar ; et
al. |
October 28, 2010 |
Particle-Optical System
Abstract
The present invention relates to a multi-beamlet multi-column
particle-optical system comprising a plurality of columns which are
disposed in an array for simultaneously exposing a substrate, each
column having an optical axis and comprising: a beamlet generating
arrangement comprising at least one multi-aperture plate for
generating a pattern of multiple beamlets of charged particles, and
an electrostatic lens arrangement comprising at least one electrode
element; the at least one electrode element having an aperture
defined by an inner peripheral edge facing the optical axis, the
aperture having a center and a predetermined shape in a plane
orthogonal to the optical axis; wherein in at least one of the
plurality of columns, the predetermined shape of the aperture is a
non-circular shape with at least one of a protrusion and an
indentation from an ideal circle about the center of the
aperture.
Inventors: |
Platzgummer; Elmar; (Wien,
AT) ; Stengl; Gerhard; (Wernberg, AT) ;
Falkner; Helmut; (Schoenau an der Triesting, AT) |
Correspondence
Address: |
Bruce D. Riter
101 First Street PMB 208
Los Altos
CA
94022-2778
US
|
Assignee: |
Carl Zeiss SMS GmbH
Jena
DE
|
Family ID: |
36061724 |
Appl. No.: |
11/990067 |
Filed: |
August 8, 2006 |
PCT Filed: |
August 8, 2006 |
PCT NO: |
PCT/EP2006/007849 |
371 Date: |
May 12, 2010 |
Current U.S.
Class: |
250/396R |
Current CPC
Class: |
B82Y 40/00 20130101;
H01J 37/12 20130101; B82Y 10/00 20130101; H01J 37/3177
20130101 |
Class at
Publication: |
250/396.R |
International
Class: |
H01J 37/12 20060101
H01J037/12; H01J 3/18 20060101 H01J003/18 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 9, 2005 |
EP |
05017318.6 |
Claims
1. A multi-beamlet multi-column particle-optical system comprising
a plurality of particle-optical multi-beamlet columns, the
plurality of particle-optical multi-beamlet columns being disposed
in an array for simultaneously exposing a same substrate; with each
particle optical column having an optical axis and comprising: a
beamlet generating arrangement comprising at least one
multi-aperture plate having a plurality of apertures for generating
a pattern of multiple beamlets of charged particles, and an
electrostatic lens arrangement downstream of the beamlet generating
arrangement, the electrostatic lens arrangement comprising at least
one electrode element, the at least one electrode element having an
aperture allowing the generated multiple beamlets of charged
particles to pass through, the aperture being defined by an inner
peripheral edge facing the optical axis, the aperture having a
centre and a predetermined shape in a plane orthogonal to the
optical axis; wherein in at least one of the plurality of charged
particle columns, the predetermined shape of the aperture of the at
least one electrode element is a non-circular shape with at least
one of a protrusion and an indentation from an ideal circle about
the centre of the aperture, and wherein a first distance between a
point on the inner peripheral edge of the aperture disposed closest
to the centre of the aperture is at least about 5% smaller than a
second distance between a point on the inner peripheral edge of the
aperture disposed furthest away from the centre of the
aperture.
2. The particle-optical system according to claim 1, wherein the
electrode element comprises an annular inner electrode member
having an inner peripheral edge and wherein the aperture is formed
by the inner peripheral edge.
3. The particle-optical system according to claim 1, wherein the
electrostatic lens arrangement of the at least one of the plurality
of charged particle multi-beamlet columns comprises two or more
electrode elements which are disposed coaxially and spaced apart in
the direction of the optical axis.
4. The particle-optical system according to claim 3, wherein the
apertures of the at least two electrode elements have substantially
the same non-circular shape.
5. The particle-optical system according to claim 4, wherein the
aperture of a first electrode element of the at least two electrode
elements has an area that is by at least 5% larger than an area of
the aperture of a second electrode element of the at least two
electrode elements.
6. The particle-optical system according to claim 1, the plurality
of columns comprising a first group of columns comprising at least
one column, wherein the aperture of the at least one electrode
element of the electrostatic arrangement of each column of the
first group of columns has a first shape, and further comprising a
second group of columns comprising at least one column, wherein the
aperture of the at least one electrode element of the electrostatic
arrangement of each column of the second group of columns has a
second shape that is different from the first shape.
7. The particle-optical system according to claim 6, wherein the
first shape is different from the second shape with respect to at
least one of a number of indentations, a number of protrusions, a
shape of a protrusion, a shape of an indentation, a size of a
protrusion, a size of an indentation, a symmetry of the shape and
any combination thereof.
8. The particle-optical system according to claim 6, wherein each
column of the first group of columns is surrounded by a first
configuration of neighbouring columns and each column of the second
group of columns is surrounded by a second configuration of
neighbouring columns, and wherein the first configuration is
different from the second configuration.
9. The particle-optical system according to claim 8, wherein the
first configuration differs from the second configuration with
respect to at least one of a number of neighbouring columns
disposed closest to the respective column, a number of neighbouring
columns disposed second closest to the respective column, a
symmetry of the configuration of neighbouring columns and any
combination thereof.
10. The particle-optical system according to claim 1, wherein the
non-circular shape comprises a shape having one, two or four
indentations extending from an ideal circle towards the centre of
the aperture, the ideal circle having a radius equal to the second
distance.
11. The particle-optical system according to claim 1, wherein each
column has at least one closest neighbouring column and at least
one second closest neighbouring column, and wherein a number of
indentations in the shape of the aperture of the at least one
electrode element of at least one column is equal to a number of
second closest columns around the at least one column, with the
indentations extending from the ideal circle towards the centre of
the aperture, the ideal circle having a radius equal to the second
distance.
12. The particle-optical system according to claim 1, wherein the
particle-optical multi-beam columns are arranged in a rectangular
array of N rows 1 to N and M lines 1 to M orthogonal to the rows,
wherein a third group of columns is comprised of columns disposed
in line 1, rows 2 to N-1, and line M rows 2 to N-1, and in row 1,
lines 2 to M-1 and row N, lines 2 to N-1, and wherein the apertures
of the at least one electrode element of the electrostatic
arrangement of each column of the third group of columns have a
same third shape.
13. The particle-optical system according to claim 1, wherein the
particle-optical multi-beam columns are arranged in a rectangular
array of N rows 1 to N and M lines 1 to M orthogonal to the rows,
wherein a fourth group of columns is comprised of columns disposed
in line 1, row 1, in line 1, row N, in line M, row 1 and in line M
row N, and wherein the apertures of the at least one electrode
element of the electrostatic arrangement of each column of the
fourth group of columns have a same fourth shape.
14. The particle-optical system according to claim 1, wherein the
particle-optical multi-beam columns are arranged in a rectangular
array of N rows 1 to N and M lines 1 to M orthogonal to the rows,
wherein a fifth group of columns is comprised of columns disposed
in lines 2 to M-1 in respective rows 2 to N-1, and wherein the
apertures of the at least one electrode element of the
electrostatic arrangement of each column of the fifth group of
columns have a same fifth shape.
15. The particle-optical system according to claim 1, wherein the
at least one electrode elements of the electrostatic arrangements
of neighbouring columns have a same distance from a substrate plane
and are arranged on a mounting structure extending substantially in
a plane orthogonal to the optical axes of the neighbouring
columns.
16. The particle-optical system according to claim 1, further
comprising a first and a second mounting structure, wherein the
optical axes of columns disposed adjacent to one another are
arranged in parallel, wherein each of the adjacent columns
comprises two or more electrode elements with a first electrode
element and at least a second electrode element being arranged
coaxially and spaced apart in the direction of the optical axis of
the column, wherein the first electrode elements of the adjacent
columns have a same first distance from a substrate plane and are
arranged on the first mounting structure, wherein the second
electrode elements of the adjacent columns have a same second
distance from the substrate plane and are arranged on the second
mounting structure, the first and second mounting structures being
arranged parallel to one another in a plane orthogonal to the
optical axes of the columns.
17. The particle-optical system according to claim 16, wherein the
first and second mounting structures are spaced apart by
electrically insulating spacer elements.
18. The particle-optical system according to claim 3, wherein the
electrode elements further comprise a substantially cylindrical
shielding member, the substantially cylindrical shielding member
having a radius equal to or greater than the second distance.
19. The particle-optical system according to claim 1, wherein the
aperture is shaped and arranged such as to provide multi-pole
correction for electrostatic fields generated within the electrode
arrangement.
20. The particle-optical system according to claim 1, wherein each
beamlet generating arrangement comprises a charged particle source
for generating a beam of charged particles, and a beam patterning
structure downstream of the charged particle source, the beam
patterning structure comprising at least the multi-aperture plate
and being configured to blank out at least a portion of the charged
particle beam such that a pattern of multiple beamlets is formed
downstream of the patterning structure.
21. The particle-optical system according to claim 1, wherein the
centre of the at least one aperture of the electrode element of the
electrostatic lens arrangement of a first column is disposed at
least 50 mm from the centre of the at least one aperture of the
electrode element of the electrostatic lens arrangement of a second
column, with the second column being arranged closest to the first
column in the array of multi-beam charged particle columns.
22. The particle-optical system according to claim 1, wherein the
shape of the aperture is asymmetric with respect to the optical
axis of the respective multi-beam particle-optical column.
23. The particle-optical system according to claim 1, wherein the
centre of the aperture is disposed on the optical axis of the
respective multi-beamlet particle-optical column.
24. A method of exposing a substrate by multi-beam multi-column
exposure, comprising: generating a plurality of multiple beamlet
patterns by a respective plurality of multi-beamlet
particle-optical columns of the particle-optical system according
to claim 1 and directing the plurality of multiple beam let
patterns towards a substrate to be exposed; generating
electrostatic fields by applying electric potentials to the at
least one electrode elements of the electrostatic lens arrangements
of the plurality of multi-beamlet particle-optical columns,
transmitting the multiple beamlet patterns through respective
apertures of the electrode elements of the electrostatic lens
arrangements of the plurality of multi-beamlet particle-optical
columns.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The invention relates to a multi-beamlet multi-column
particle-optical system, in particular a multi-beamlet multi-column
particle-optical system comprising a plurality of multi-beamlet
particle-optical columns wherein at least one of the multi-beamlet
particle-optical columns comprises an electrode element having an
aperture of a noncircular shape. The invention further relates to a
method of exposing a substrate by multi-beam multi-column exposure
using the multi-beamlet multi-column particle-optical system.
[0003] 2. Brief Description of Related Art
[0004] The increasing demand for ever smaller and more complex
microstructured devices and the continuing demand for an increase
of a throughput in the manufacturing and inspection processes
thereof have been an incentive for the development of
particle-optical systems that use multiple charged particle
beamlets in place of a single charged particle beam, thus
significantly improving the throughput of such systems. The use of
multiple beamlets is associated with a whole range of new
challenges to the design of particle-optical components,
arrangements and systems, such as microscopes and lithography
systems.
[0005] A particle-optical arrangement for forming a plurality of
charged-particle beamlets wherein the beamlets are arranged in an
array pattern is described in U.S. Pat. Nos. 5,369,282 and
5,399,872, for instance.
[0006] Multi-beamlet particle-optical systems make use of a pattern
of multiple charged particle beamlets focused on a substrate to be
exposed. For example, in an inspection system, a single beam of
charged particles is provided by a particle source or,
alternatively, multiple beamlets may be provided by an array of
charged particle sources. The beam or beamlets is/are then
typically directed onto a multi-aperture plate having a plurality
of apertures formed therein for generating multiple beamlets from
those charged particles of the single beam or beamlets that pass
through the apertures of the multi-aperture plate. The multiple
beamlets are generally subsequently focused on the substrate,
typically by means of a focussing particle-optical lens downstream
of the multi-aperture plate. An array of charged particle spots is
thus formed on the substrate. Secondary charged particles such as
secondary electrons may be emitted by the substrate to be
inspected, follow a secondary beamlet path and are incident on a
detector.
[0007] Further more, in particle-optical lithography, methods of
so-called maskless lithography have been established, which, for
instance, make use of a blanking aperture array. Such a blanking
aperture array typically comprises a multi-aperture plate wherein
each of a plurality of apertures is further equipped with a
deflecting arrangement, generally comprising electrodes, which, in,
a "switched-on" or activated state, is capable of deflecting a
beamlet passing through the respective aperture such that it is
deflected from a beam path of the beamlets to such an extent that
it does not reach the specimen and does not contribute to an
exposure of the substrate or specimen. The deflecting arrangements
of the individual apertures can thus be switched off or
de-activated to let a beamlet pass undisturbed through the
respective aperture and a switched-on state where a passing beamlet
is deflected away from a beam path and incident on an obstacle in
the form of a non-transmitting portion of an aperture or the like
such that it will not be incident onto the specimen. By suitable
movement of the blanking aperture array relative to the specimen to
be exposed and suitable switching sequences of the individual
apertures, a pattern can be generated and written onto the
specimen, such as described, for instance in US 2003/0155534 A1,
the entire content of which is incorporated by reference
herein.
[0008] In addition to using a plurality of beamlets, systems
employing a plurality of two or more particle-optical systems, or
columns, operating in parallel to simultaneously expose or inspect
the same substrate are being developed. Given that, due to
interactions of charged particles, a throughput and a performance
of an individual particle-optical system (column) is generally
limited by a maximum acceptable current of charged particles in the
system, the multi-column approach allows to increase the throughput
of such a particle-optical lithography system without further
increasing the current through an individual column and therefore
avoids a decrease in performance due to space charge effects. Thus,
multi-column particle-optical systems comprise a plurality of
particle-optical columns which each, in terms of their components
and their arrangement, largely correspond to a conventional
particle-optical system as described above. An example of a
multi-column particle-optical system is described in US Patent
Application with publication number US 2005/0104013 A1, the entire
content of which is incorporated by reference herein.
[0009] Using an array or pattern of beamlets of charged particles
requires a multi-beamlet particle-optical system to provide those
beamlets in a reliable and accurate manner such that the individual
beamlets show little, if any, variation in intensity, deviation
from a predetermined position within the array and target position
on a substrate, variation in optical properties, such as
aberrations and the like. The quality of the pattern of beamlets
and, correspondingly, the quality of the pattern of charged
particle spots generated in an image or substrate plane,
respectively, will generally depend, amongst others, on properties
of the beamlet generating arrangement used as well characteristics
of the focussing arrangement, such as a lens.
[0010] In addition, external factors originating in an environment
of the particle-optical system may also influence a performance of
the particle-optical system. An example of such an external factor
exerting a negative influence on an imaging performance are
electromagnetic fields from outside the charged particle system
penetrating into the system, as discussed, for instance in US
2005/0072933 A1, the entire content of which is incorporated by
reference herein. The electrostatic lens system described therein
comprises an electrostatic lens arrangement having more than three
electrode elements which are arranged coaxially in series along an
optical axis of the electrostatic lens arrangement. Additional
shielding is provided by provision of an outer member ring to fill
a space between two adjacent electrode elements thus preventing
intrusion of interfering electromagnetic fields. The system
described therein is a single column system.
[0011] In multi-column systems, additional problems arise from the
close arrangements of individual columns and their electrostatic
and/or electromagnetic fields, which may cause interferences in
neighbouring columns. These interferences, for instance a
disturbance of a focussing electrostatic and/or electromagnetic
field, may cause imaging errors such as particle-optical
aberrations and thus deteriorate an imaging performance.
[0012] It is therefore an object of the present invention to
provide a multi-beamlet multi-column particle-optical system
providing an improved imaging performance.
[0013] It is a further object of the present invention to provide a
multi-beamlet multi-column particle-optical system configured to
decrease an influence from one or more neighbouring columns on an
imaging performance.
[0014] It is another object to provide an improved method of
multi-beam multi-column particle-optical exposure.
SUMMARY OF THE INVENTION
[0015] As will be described in more detail in the following, the
present invention provides a multi-beamlet multi-column
particle-optical system comprising a plurality of particle-optical
multi-beamlet columns; the plurality of particle-optical
multi-beamlet columns being disposed in an array for simultaneously
exposing a same substrate; with each particle optical column having
an optical axis and comprising: a beamlet generating arrangement
comprising at least one multi-aperture plate having a plurality of
apertures for generating a pattern of multiple beamlets of charged
particles, and an electrostatic lens arrangement downstream of the
beamlet generating arrangement, the electrostatic lens arrangement
comprising at least one electrode element, the at least one
electrode element having an aperture allowing the generated
multiple beamlets of charged particles to pass through, the
aperture being defined by an inner peripheral edge of the electrode
element facing the optical axis, the aperture having a centre and a
predetermined shape in a plane orthogonal to the optical axis;
wherein in at least one of the plurality of charged particle
columns, the predetermined shape of the aperture of the at least
one electrode element is a non-circular shape with at least one of
a protrusion and an indentation from an ideal circle about the
centre of the aperture, and wherein a first distance between a
point on the inner peripheral edge of the aperture disposed closest
to the centre of the aperture is at least about 5%, preferably at
least about 7.5% or more preferably at least about 10% smaller than
a second distance between a point on the inner peripheral edge of
the aperture disposed furthest away from the centre of the
aperture.
[0016] The provision of an electrode element having a non-circular
aperture allows to correct disturbances of an electrostatic and/or
magnetic field within the electrostatic lens arrangement in the
column comprising the electrode element having the non-circular
aperture. The disturbances or interferences may be caused by
neighbouring columns, in particular. The shape of the aperture is
adapted to compensate for those disturbances, and optionally also
other disturbances, by superimposing additional shape features onto
a basic circular shape. The superimposed shape features, in
particular indentations and protrusions from an ideal circular
shape, allow to generate correcting electrostatic fields, such as
multipole electrostatic fields, for instance, in addition to a
basic electrostatic field generated by an electrode element having
a basic, round shape. Thus, a suitable design of the shape of the
aperture of the electrode element allows to correct aberrations
caused by an electrostatic/electromagnetic field in a beam path of
the charged particle beamlets deviating from an ideal
electrostatic/electromagnetic field as a result of disturbances or
interferences, such as electrostatic and/or electromagnetic fields
of neighbouring columns. Preferably, in terms of disturbances, the
interferences generated by an adjacent electrode element, or
generally electrostatic field generating arrangement, in the same
plane as a given electrode element, as well as interferences
generated by an adjacent electrode element disposed in a different
plane, for instance upstream or downstream of the given electrode
element, are taken into consideration. In other exemplary
embodiments, only the interferences resulting from electrode
elements, or more generally electrostatic field generating
arrangements, in the same plane as the given electrode element my
be considered when optimising the non-circular shape of the
aperture of the given electrode element.
[0017] An indentation, as used herein, is a shape feature that
extends from a (virtual) ideal circular shape towards the centre of
the shape whereas a protrusion extends from a (virtual) ideal
circular shape towards the outside of the ideal circle, i.e. away
from its centre. In other words, for purposes of describing a
non-circular shape, each shape is associated with an ideal circle
having shape features superimposed thereto. Generally, out of a
number of possible circles, the ideal circle would be chosen such
that it comprises most of the points disposed on the peripheral
edge of the shape. It may also be chosen such that the area covered
by the shape features is minimal, for instance in cases where a
shape may be described as having indentations or may be equally
described as having protrusions. In that case the ideal circle can
be advantageously chosen such that the (absolute) area covered by
the respective shape feature is minimal, and in case that the areas
of the indentations or protrusions should be equal, the area of the
ideal circle may be chosen to be minimal.
[0018] In exemplary embodiments, the first distance is by at least
about 5% smaller than the second distance, in further exemplary
embodiments by at least about 10%, for instance at least about 15%
or 20% smaller. The difference between the first and second
distances is indicative of a deviation of the shape of the aperture
from the ideal circular shape, or a contribution of additional
shape features, which difference enables the provision of a desired
field shaping or field correcting effect.
[0019] The difference between the first distance and the second
distance may be, in an exemplary embodiment, at least 1 .mu.m, or
may be at least 5 .mu.m, or more preferably at least 10 .mu.m and
in further exemplary embodiments at least 15 .mu.m.
[0020] The at least one electrode element may be comprised of only
one part or several parts or members.
[0021] Inner peripheral edge, as used herein, may be a protruding
portion of the electrode element or a member thereof, for instance,
or may be a portion of a substantially continuous inner side of the
electrode element or a member thereof, or any other suitable
configuration.
[0022] Preferably, the centre of the aperture is disposed on the
optical axis of the multi-beamlet particle-optical column
comprising the aperture.
[0023] Generally, the centre of the aperture is the point of the
aperture having the highest symmetry. The shape of the aperture is
generally asymmetric with respect to the centre thereof. In
exemplary embodiments, the shape of the aperture of one or more
columns may be rotationally asymmetric.
[0024] Preferably, the optical axes of the columns of the plurality
of columns are arranged in parallel.
[0025] Downstream, as used herein, refers to a direction of charged
particles in the particle-optical system, starting at the beamlet
generating arrangement or, more generally, a charged particle
source and ending at the substrate (or specimen) plane.
[0026] Exposing the substrate may comprise exposing the substrate
to charged particles to write a pattern onto a substrate in a
lithographic process or to inspect the substrate by detecting
secondary charged particles emitted by the substrate as a result of
charge particles impinging onto a surface thereof.
[0027] In exemplary embodiments, the aperture of the electrode
element may be formed by the inner peripheral edge of an annular
inner electrode member. The inner peripheral edge and/or the entire
annular inner electrode member (that is an inner side and an outer
side of the member) may taper or extend under an angle towards the
optical axis. In those embodiments, the noncircular shape of the
aperture may extend over the entire axial dimension of the inner
electrode member or just an axial portion of the inner electrode
member and inner peripheral edge, respectively. The axial portion
in those exemplary embodiments may be, for instance, a portion of
the inner electrode member disposed closest to the optical axis,
i.e. a portion where the area of the aperture formed by the inner
peripheral edge of the electrode member is smallest and the inner
peripheral edge hence closest to the optical axis. In further
exemplary embodiments, the inner electrode member has a
substantially conical shape with the inner peripheral edge on the
inside of the conus forming a noncircular aperture at least over an
axial portion, preferably over an entire axial length of the inner
electrode member.
[0028] The cross-section of the inner electrode member may have a
regular or an irregular shape, for instance in the axial direction.
In exemplary embodiments, the inner electrode member may, for
instance, be split or forked into two portions each extending
towards the optical axis, for instance at two different angles. In
other exemplary embodiments, the inner electrode member may be
constituted by or at least comprise a protruding portion extending
towards the optical axis and having a curved shape, the curved
shape, for instance being a regular or irregular concave shape as
seen from a direction of beamlets of particles passing through. The
inner electrode member and/or a protruding portion thereof may have
a constant thickness or a varying thickness.
[0029] In preferred embodiments, and in line with the teaching of
US 2005/0072933 A1, as mentioned above, in the at least one of the
plurality of charged particle multi-beamlet columns, the
electrostatic lens arrangement comprises two or more electrode
elements which are disposed coaxially and spaced apart, i.e. in
series, in the direction of the optical axis. Providing a series of
coaxially aligned electrode elements allows improved shielding of a
space inside the electrode elements, as mentioned in connection
with the disclosure of the above-cited patent application. In
exemplary embodiments, three or more than three electrode elements
are provided. In further exemplary embodiments, the majority or all
of the plurality of multi-beamlet particle-optical columns comprise
two or more electrode elements, for instance three or four and more
electrode elements. Each electrostatic lens arrangement of the
plurality of multi-beamlet particle-optical columns may comprise
the same number of electrode elements. Further more, the
electrostatic lens arrangements of the plurality of multi-beamlet
particle-optical columns may be substantially the same or may be
different in terms of configuration, such as a number of electrode
elements, a distance of electrode elements from one another, a
position of the electrostatic lens arrangement within the column
and other parameters, as will be readily apparent to the skilled
person.
[0030] In those embodiments wherein the electrostatic lens
arrangement of the at least one of the plurality of particles
comprises two or more electrode elements, the apertures of the at
least two electrode elements (of the same, at least one column)
preferably have substantially the same non-circular shape. In other
embodiments, the at least two electrode elements may have different
non-circular shapes. A difference in shapes may relate, for
instance, to at least one of a number of indentations, a number of
protrusions, a shape of one or more protrusions, a shape of one or
more indentations, a size of one or more protrusions, a size of one
or more indentations, a symmetry of the shape and any combination
thereof. It is conceivable, for instance, that each electrode
element may comprise one specific shape feature, such as an
indentation or protrusion, that is added to the basic circular
shape to contribute a specific component to an electrostatic field,
such as a specific component of a multipole field.
[0031] For instance, the apertures of the at least two electrode
elements may have areas that differ by at least 5%, based on a
largest area. Thus, if at least two electrode elements and thus at
least two apertures are provided, an aperture of a first electrode
element may be at least 5% larger than an aperture of a second
electrode element. In those embodiments, the first electrode
element would preferably be disposed downstream of the second
electrode element. In general, if a plurality of electrode elements
are provided spaced apart along the optical axis in an
electrostatic lens arrangement, areas of apertures formed by
peripheral inner edges thereof may be substantially equal, i.e.
substantially constant along the optical axis, or alternatively,
may increase or decrease in a downstream direction.
[0032] In exemplary embodiments of the particle-optical system
according to the present invention, the plurality of columns
comprises a first group of columns comprising at least one column,
wherein the aperture of the at least one electrode element of the
electrostatic arrangement of each column of the first group of
columns has a first shape, and further comprises a second group of
columns comprising at least one column, wherein the aperture of the
at least one electrode element of the electrostatic arrangement of
each column of the second group of columns has a second shape that
is different from the first shape. In particular, the first shape
may be different from the second shape with respect to at least one
of a number of indentations, a number of protrusions, a shape of a
protrusion, a shape of an indentation, a size of a protrusion, a
size of an indentation, a symmetry of the shape, and any
combination thereof, as mentioned above.
[0033] In those embodiments, preferably each column of the first
group of columns is surrounded by a first configuration of
neighbouring columns and each column of the second group of columns
is surrounded by a second configuration of neighbouring columns,
wherein the first configuration is different from the second
configuration. The first configuration may differ from the second
configuration with respect to at least one of a (total) number of
neighbouring columns disposed closest to the column, a distance of
a neighbouring closest column, a (total) number of neighbouring
columns disposed second closest to the column, a distance of second
closest neighbouring columns, a symmetry of the configuration of
neighbouring columns, and any combination thereof. The same
applies, of course, to any other configurations of neighbouring
columns that differ from one another.
[0034] An influence exerted by neighbouring columns on an
electrostatic field within a certain column will typically depend
on the arrangement or configuration of neighbouring columns around
said column, in particular the number of neighbouring columns
around said column, their distance from said column, the symmetry
of the configuration, the number of in particular the nearest,
second nearest and, potentially, further order nearest columns, and
the like. Therefore, for a given configuration of neighbouring
columns around any particular column, an influence exerted by the
given configuration may be determined, for instance either
experimentally or by calculation or simulation, and a non-circular
shape of the electrode element of a particular column adapted and
optimized accordingly. Thus, each configuration of columns around
said particular column may be associated with a predetermined
non-circular shape of the aperture of the at least one electrode
element of said particular column. For instance, in an array of
columns, the first group of columns may comprise those that are
surrounded by neighbouring columns on all sides whereas the second
group of columns may comprise those columns that are disposed on an
edge of the array and thus generally would not have columns
disposed on all of their sides. The columns of the first group may,
in such a configuration, be exposed to symmetric external
disturbances whereas the columns of the second group would
typically be exposed to asymmetric disturbances. The shapes of the
apertures in the columns in the different groups of columns may
therefore be optimised with regard to this difference in
configurations and its effect on extent, location and symmetry of
disturbances.
[0035] In exemplary embodiments of the particle-optical system
according to the present invention, the non-circular shape
comprises a shape having one, two or four indentations extending
from an ideal circle towards the centre of the aperture, the ideal
circle having a radius equal to the second distance (wherein the
centre of the ideal circle coincides with the centre of the
aperture).
[0036] Generally, each column has at least one closest neighbouring
column and at least one second closest neighbouring column. In
further exemplary embodiments, the shape of the aperture of the
electrode element of a respective column, preferably of each column
of the plurality of columns, has a number of indentations extending
from the ideal circle towards the centre of the aperture, which
number is equal to a number of second closest columns around the
respective column, with the ideal circle having a centre coinciding
with the centre of the aperture and a radius equal to the second
distance. In those embodiments where the array of the plurality of
columns takes the form of a regular, rectangular grid of columns,
for instance, an aperture of a column disposed on a corner of the
grid would have one indentation only, in accordance with this
exemplary embodiment, whereas apertures in columns disposed on an
edge, but not in a corner-position would have two indentations and
all apertures of columns surrounded by columns on all sides would
have four indentations.
[0037] Alternatively or in addition to the above embodiments, the
particle-optical multi-beam columns may, in a further exemplary
embodiment, be arranged in a rectangular array of N rows 1 to N and
M lines 1 to M orthogonal to the rows, wherein a third group of
columns is comprised of columns disposed in line 1, rows 2 to N-1,
and line M rows 2 to N-1, and in row 1, lines 2 to M-1 and row N,
lines 2 to N-1, and wherein the apertures of the at least one
electrode element of the electrostatic arrangement of each column
of the third group of columns have a same third shape. In addition
or as an alternative to this exemplary embodiment, in an array of
columns as laid out above, a fourth group of columns may be
comprised of columns disposed in line 1, row 1, in line 1, row N,
in line M, row 1 and in line M row N, wherein the apertures of the
at least one electrode element of the electrostatic arrangement of
each column of the fourth group of columns have a same fourth
shape. In addition or as an alternative there to, in an array of
columns as laid out above, a fifth group of columns may be
comprised of columns disposed in lines 2 to M-1 in respective rows
2 to N-1, wherein the apertures of the at least one electrode
element of the electrostatic arrangement of each column of the
fifth group of columns have a same fifth shape. Thus, apertures of
electrode elements in columns on a corner, on a periphery and
inside the array of columns will differ with respect to
non-circular shapes, for instance with respect to at least one of a
number of indentations, a number of protrusions, a shape of a
protrusion or indentation, a size of a protrusion or an
indentation, a symmetry of the shape and any combination thereof,
as described before.
[0038] These shapes of the apertures of the embodiments described
above are, evidently, based on a simplified evaluation of
disturbances from neighbouring columns. The shape of the aperture
of the at least one electrode element of each individual column
may, of course, be adapted individually to account for a total
number of neighbouring columns, their configuration and the like,
to compensate for higher order disturbances, i.e. disturbances
generated by columns spaced further apart, rather than the
disturbances that take only the first and second closest columns
and their configuration into account. This may be achieved, for
instance, by adding further shape features or may altering a size,
position, or shape of at least one of a protrusion and an
indentation.
[0039] In preferred embodiments of the particle-optical system
according to the present invention, at least two columns,
preferably all of the columns of the plurality of columns, have
electrostatic lens arrangements with an electrode element comprised
therein which is disposed in a plane spaced at a predetermined
distance from a substrate plane, with said plane preferably being
arranged parallel to the substrate plane. Further, in those
preferred embodiments, the electrode elements which are comprised
in neighbouring columns and which are disposed in said plane may be
arranged on a common mounting structure, which mounting structure
preferably would be substantially disposed in or at least parallel
to said plane, and preferably orthogonal to the optical axes of the
neighbouring columns. An individual electrode element or at least a
portion thereof, such as an annular inner member, as referred to
above, may be inserted into, fixed to, rested on or otherwise held
by the mounting structure. For instance, the mounting structure may
be an electrode element mounting plate, which mounting plate may
comprise openings for the individual electrode elements or
portions/members thereof. The electrode elements may be
manufactured independently from the mounting structure or be
integrally formed with the mounting structure. Use of a mounting
structure has an advantage in that it facilitates positioning
electrode elements of electrostatic lens arrangement of
neighbouring columns to be suitably positioned, in particular
positioned such that they all have the same distance from the
substrate plane and are thus disposed in the same plane.
[0040] In those embodiments of the particle-optical system
according to the present invention wherein electrostatic lens
arrangements of two or more adjacent columns each comprise two or
more electrode elements, in particular a first electrode element
and a second electrode element arranged coaxially to the first
electrode element and spaced apart in the direction of the optical
axis, the first electrode elements of the electrostatic
arrangements of adjacent columns may be disposed at a same first
distance from the substrate plane and mounted to a first mounting
structure and the respective second electrode elements of the
electrostatic arrangements of columns may be disposed at a same
second distance from the substrate plane and mounted on a second
electrode element mounting structure. The first and second mounting
structures are then arranged parallel to one another in the plane
orthogonal to the optical axis. Likewise, respective third
electrode elements may be arranged on a common third mounting
structure, and further electrode elements likewise.
[0041] In those embodiments, the first and second mounting
structures may be spaced apart by electrically insulating spacer
elements, which may be disposed at a periphery of the mounting
structures or, optionally, at peripheries of the respective first
electrode elements.
[0042] In particular in those embodiments of the present invention
wherein the electrostatic lens arrangement of at least one column
comprises two or more electrode elements, the electrode elements
may, in addition to an annular inner electrode member, for
instance, comprise a substantially cylindrical shielding member,
the substantially cylindrical shielding member preferably having a
radius equal to or greater than the second distance. The shielding
member may be disposed in a gap between electrode elements disposed
adjacent to one another along the optical axis so as to further
prevent interferences from an outside of the respective column in a
space inside the electrostatic lens arrangement. The shielding
member may be a cylinder, which is arranged coaxially with the
optical axis and extends at least over an axial portion of the gap
between respective portions of adjacent electrode elements.
Dimensions and location of said portion would be chosen such that
no electrical shortcuts are generated in between the adjacent
electrode elements. In those embodiments of the present invention
wherein first electrode elements are mounted on a first mounting
structure and second electrode elements on a second mounting
structure, and wherein the electrode elements comprise at least an
annular inner electrode member and a shielding member, the inner
electrode member may be disposed such as to substantially extend on
one side of the mounting structure, as seen in a direction of the
optical axis, and the shielding element such as to extend
substantially on the other (opposite) side of the mounting
structure. For instance, the shielding element may be disposed
substantially upstream of the mounting structure and the inner
electrode member such that it extends substantially downstream of
the mounting structure.
[0043] Preferably, in all embodiments of the present invention, the
aperture of an electrode element is shaped and arranged such as to
provide multi-pole correction for electrostatic fields generated
within the electrostatic lens arrangement the electrode element is
comprised in.
[0044] In exemplary embodiments of the particle-optical system
according to the present invention, each beamlet generating
arrangement comprises a charged particle source for generating a
beam of charged particles and a beam patterning structure
downstream of the charged particle source, which comprises the
multi-aperture plate, and optionally further components, and which
is configured to blank out at least a portion of the charged
particle beam such that a pattern of multiple beamlets is formed
downstream of the patterning structure.
[0045] The charged particle source may be any conventional particle
source suitable for use in the present invention. In those
embodiments where the charged particles are electrons, the charged
particle source would be an electron source, such as an electron
source of a thermal field emission (TFE) type. In those embodiments
where ions are used as charged particles, an ion gun would be a
suitable charged particle source, for instance. Charged particle
sources suitable for use in the present invention are well known in
the art and include sources employing a tungsten (W) filament,
LaB.sub.6 sources and various others. The charged particle source
may be, for instance, a source of a single beam of charged
particles, may be an array of sources of a single charged particle
beam each, or a multi-beam source.
[0046] In typical embodiments of the particle-optical system
according to the present invention, a centre of the at least one
aperture of the electrode element of the electrostatic lens
arrangement of a first column is disposed at least 50 mm from a
centre of the at least one aperture of the electrode element of the
electrostatic lens arrangement of a second column, with the second
column being arranged closest to the first column in the array of
multi-beam charged particle columns. More generally, optical axes
of neighbouring columns may be in a range of from about 75 mm to
about 100 mm spaced apart.
[0047] Moreover, in preferred embodiments, one or more of the
plurality of multi-beamlet columns comprises a voltage source for
applying a potential to the at least one electrode element of their
electrostatic lens arrangements. If the electrostatic lens
arrangement comprises more than one electrode element, each
electrode element may have a different potential applied thereto.
Each column in the column array may have its own one or more
voltage supplies, or alternatively, two or more columns in the
column array may share one or more voltage supplies. Preferably,
the voltage supply is an adjustable voltage supply.
[0048] Electrode elements having an aperture having a non-circular
shape could be suitably manufactured by casting a electrode element
with a circular aperture which is subsequently turned on a special
high-precision lathe to create the additional shape features, i.e.
finely process the inner peripheral edge in a radial and optionally
in an axial direction to create the desired non-circular shape. The
special lathe comprises a first and additionally a second tool-feed
axis that is superimposed to the first tool-feed axis. The second
tool-feed axis may be put into practice by means of piezoelectric
translator elements in order to reach the required accuracy.
Preferably, the accuracy of manufacturing to be achieved is in a
range of about 2 to 10 micrometers.
[0049] Those embodiments that comprise two or more mounting
structures, the mounting structures may be built as stiff cages
having ceramic insulators disposed between them. The mounting
structures may be soldered together. The electrode elements may be
machined/subjected to further fine processing after having been
mounted to the mounting structure to allow for a uniform and more
precise machining of all electrode elements mounted in the same
plane and to the same mounting structure.
[0050] The electrodes may be made from soft magnetic material, for
instance, such as soft iron or permalloy. Furthermore, the
electrodes may be coated with a suitable coating, such as thin
films of well conducting materials. In exemplary embodiments, a
thin film, such as a thin film of titanium, gold, platinum, or any
other precious metal may be provided. In alternative embodiments, a
homogeneous thin film of carbon may also be used. In addition to
the thin film, a bonding agent may be used to enhance adhesion of
the thin film to the surface of the multi-aperture plate, for
example a thin film of bonding agent may be used in between the
plate surface and the thin metal film. As an example, Cr, W or Pt,
or any suitable combination thereof, may be used as a bonding
agent. These exemplary embodiments are advantageous for protecting
the electrode surface from contaminations, in particular when a
potential is applied, and may assist in decreasing heat and/or
charge accumulating on a respective surface or avoiding oxidation
thereof.
[0051] The present invention, in a second aspect, further pertains
to a method of exposing a substrate by multi-beam multi-column
exposure, comprising: generating a plurality of multiple beamlet
patterns by a respective plurality of multi-beamlet
particle-optical columns of the particle-optical system according
to the present invention and directing the plurality of multiple
beam let patterns towards a substrate to be exposed; generating
electrostatic fields by applying electric potentials to the at
least one electrode elements of the electrostatic lens arrangements
of the plurality of multi-beamlet particle-optical columns,
allowing the multiple beamlet patterns to pass through respective
apertures of the electrode elements of the electrostatic lens
arrangements of the plurality of multi-beamlet particle-optical
columns.
[0052] Embodiments, features and effects of the method of the
present invention generally correspond to or may be derived from
the embodiments and effects described above in connection with the
system of the present invention.
[0053] Embodiments and advantages of components, in particular the
electrode elements, the apertures, the mounting structure and so
on, are in accordance to the ones described above in connection
with the multi-beamlet multi-column particle-optical system.
BRIEF DESCRIPTION OF THE DRAWINGS
[0054] The foregoing as well as other advantageous features of the
present invention should become more apparent from the following
detailed description of exemplary embodiments of the invention with
reference to the accompanying drawings.
[0055] It is noted that not all possible embodiments of the present
invention necessarily exhibit each and every, or any, of the
advantages identified herein.
[0056] FIG. 1 schematically illustrates an embodiment of one column
of a multi-beamlet multi-column particle-optical system according
to the present invention, in a cut-open view;
[0057] FIG. 2 schematically illustrates an embodiment of an
electrostatic lens arrangement for use in a system according to the
present invention;
[0058] FIG. 3 schematically illustrates an elevational view (in a
direction of the optical axes) onto an embodiment of an array of
respective first electrode elements of electrostatic lens
arrangements of neighboring columns, said first electrode elements
being mounted on a common first mounting plate;
[0059] FIG. 4 schematically illustrates an embodiment of a
configuration of apertures of neighboring lenses around the
aperture shown in FIG. 5;
[0060] FIG. 5 schematically illustrates an embodiment of a
noncircular shape of an aperture of an electrode element comprised
in an electrostatic lens arrangement according to the present
invention;
[0061] FIG. 6 schematically illustrates a cut along line A-A
through the embodiment of the array of electrode elements as
depicted in FIG. 3.
DETAILED DESCRIPTION OF DEPICTED EMBODIMENTS
[0062] In FIG. 1, particle-optical components of an embodiment of
multi-beamlet particle-optical column 1 for use in a multi-column,
multi-beamlet particle-optical system according to the present
invention are illustrated. In the depicted embodiment, column 1
comprises, in a direction in which the charged particles would
generally travel, a beamlet generating arrangement 100,
electrostatic lens 200 and electromagnetic focussing lens
arrangement 400. Beamlet generating arrangement 100 comprises
charged particle source 110, extraction system 120 and condenser
lens 130. The condenser lens 130 comprises a stack of electrodes
131. The beamlet generating system further comprises
beam-patterning structure 141 held by a mounting frame 141. The
beam patterning structure 141 generally comprises a multi-aperture
plate, and may be a blanking aperture array, for instance, as
described above. The electrostatic lens arrangement 200 comprises a
plurality of electrode elements 210 which are arranged in series
along the optical axis of the system, i.e. placed in a stack to be
traversed by the charged particles. In the depicted embodiment, the
distances between adjacent electrode elements are substantially the
same. In other embodiments, these differences may vary.
Electromagnetic focussing lens arrangement 400 comprises three
electromagnetic lenses 401, 402, 403, with electromagnetic lens 403
being disposed closest to a substrate plane S and comprising a
conical portion 403A.
[0063] Charged particle source 110 generates charged particles, and
may be, for instance, an electron gun for generating electrons.
Extraction system 120 extracts the generated charged particles,
accelerates them to a desired energy level and directs them towards
a substrate plane S. Condenser lens 130 directs and forms the
charged particles into a substantially telecentric beam of charged
particles. The beam of charged particles is then incident on beam
patterning structure 140. In case of a blanking aperture array,
charged particles pass through the apertures of the blanking
aperture arrays that are in a switched on state, thus forming a
plurality of beamlets. Details of such a blanking aperture array
may be found in US 2003/0155534 A1, as already referred to
above.
[0064] The electrostatic lens arrangement 200 depicted in FIG. 1
forms an immersion-type lens. For providing a focussing effect, an
electrostatic field is generated by applying a suitable electric
potential to at least one electrode element of the electrostatic
lens arrangement. Together with the electromagnetic focussing lens
arrangement 400, the electrostatic lens arrangement 200 forms a
projection system of the particle-optical system, which serves to
project an image of the patterning structure 140, in the case of a
blanking aperture array the switched on apertures, into the
substrate plane S to expose a substrate disposed therein with the
pattern of the switched on apertures.
[0065] FIG. 2 shows an example of an electrostatic lens arrangement
200. Electrostatic lens arrangement 200 comprises a plurality of
electrode elements 210 which are disposed coaxially in a stack
along the optical axis OA of the electrostatic lens arrangement
200. The electrostatic lens arrangement 200 further comprises a
front element 201 upstream of the stack of electrode elements 210
and an exit element 202 downstream of the stack of electrode
elements 210. An aperture formed by the front element 210 is, in
the depicted embodiment, substantially of the same size as
apertures 213 the electrode elements 210 whereas an aperture formed
by back element 202 is substantially smaller. The reduced size is
feasible since the charged particles traversing the electrostatic
lens arrangement 200 are focussed by same such that a diameter of a
beam path BP entering the electrostatic lens arrangement 200 at a
front end is larger than a diameter of the beam path exiting from
the electrostatic lens arrangement 200 through exit element 202, as
indicated by the outline of the beam path in FIG. 2. It is to be
noted that the beam path encompasses the beamlet paths of the
multiple beamlets generated by the beam patterning structure
upstream of the electrostatic lens arrangement 200, but is
illustrated as a an envelope of the beamlet paths for ease of
illustration. Front element 210 and exit element 202 serve to
define and control an initial and a final electrostatic potential
for the charged particles entering and exiting from the
electrostatic lens arrangement 200.
[0066] Adjacent electrode elements 210 are separated by spacer
elements 280. Each electrode element 210 comprises an outer ring
portion 220, an inner electrode member 211 and a connecting element
221 for connecting the outer ring portions 220 and the inner
electrode member 221. Inner electrode members, in the depicted
embodiment, have a wedge-like shape with a thicker portion at an
outer peripheral end and a thinner portion at an inner peripheral
end. Electrode element 210, in particular inner electrode member
211, has an inner peripheral edge 212 that defines an aperture 213
through the electrode element 210 and thus the inner electrode
member 211, in a plane P orthogonal to the optical axis OA. In the
depicted embodiment, the non-circular shape of the apertures 213
formed is not visible. The provision of the spacer elements 280
allows to apply an individual electrostatic potential to each
electrode element 210 independently of other potentials applied to
other electrode elements. The electrical connections for applying
potentials are not depicted in FIG. 2. Electrostatic potentials may
be supplied by one or more voltage supplies (power sources), or a
resistor array interpolating the individual electrostatic
potentials from a small number of potentials supplied, for
instance.
[0067] The electrode elements 210 depicted in FIG. 2 have
substantially the same configuration; in particular the depicted
electrode elements have apertures 213 that do not substantially
vary in size. In other embodiments, however, sizes (in particular
in terms of areas within plane P) of apertures 213 of subsequent
electrode elements may vary, for instance decrease in a downstream
direction. This would be the case, for instance if a distance of
the inner peripheral edge of each electrode element 210 was to have
about a same distance from an outer periphery of the envelope of
the multiple beamlets of charged particles, a diameter of which
decreases in a downstream direction.
[0068] In other embodiments, reference numeral 220 could be
replaced by reference numeral 212 to indicate a shape of the inner
peripheral edge in a plane P' different from, but parallel to plane
P, i.e. a plane at a different axial position, for instance
upstream of plane P. The inner peripheral edge 212 would then form
an aperture having a circular shape in the plane P' and alter in
the axial direction to form an aperture 213 having a noncircular
shape in plane P. Preferably, and in accordance with the depicted
embodiment, the aperture 213 in plane P has a smaller area than the
aperture formed in plane P'.
[0069] In FIG. 3, an array 250 of 5.times.5 electrode elements 210
is depicted, with the individual electrode elements 210 being held
by a common mounting plate 270. Each individual electrode element
210 is part of an individual column of the plurality of columns,
which are, in correspondence to their electrode elements, arranged
in the 5.times.5 array. The array of electrode elements 210 is a
regular rectangular array having rows 1 to N, in this case rows 1
to 5 and lines 1 to M, herein lines 1 to 5. As described before,
each electrode element 210 has an aperture 213 having a noncircular
shape. Each aperture 213 is formed by inner peripheral edge 212 of
electrode element 210. Inner edges of cylindrical outer members 220
are also visible in FIG. 3. In the depicted embodiment the
non-circular shapes comprise at least one indentation I from an
ideal circular shape. The shapes of the apertures 213 are dependent
on a position of the respective electrode element 210 within the
array 250. The apertures 213 of electrode elements 210 of a first
group of columns) disposed in positions (row, line) (1,1), (N, 1),
(1, M) and (N, M) have a shape with one indentation I each, the
respective indentation I facing away from a respective corner of
the array. The apertures 213 of electrode elements 210 of a second
group of columns disposed in positions (row, line) (2-4, 1), (1,
2-4), (2-4, M) and (N, 2-4) have substantially the same
non-circular shape (excluding the orientation thereof) having two
indentations I from an ideal circular shape each. The apertures 213
of electrode elements 210 of a third group of columns disposed in
positions (row, line) (2-4, 2-4) have substantially a same shape
with four indentations I from an ideal circular shape. The
difference between the apertures of the respective groups is the
number of nearest and second nearest neighbours and the symmetry of
the configuration of neighbouring columns. The apertures of the
first group of columns are disposed on corners of the array and
have two nearest and one second nearest neighbour. Electrostatic
fields generated by such an environment may be compensated for by
the provision of one indentation oriented in a direction of the
second nearest neighbouring column. The columns disposed on edges
of the array, but not on corners have neighbouring columns in three
directions with three nearest and two second nearest neighbouring
columns each. Electrostatic fields generated by such an environment
may be suitably compensated four by providing the respective
apertures 213 with a non-circular shape having two indentations I
from an ideal circular shape, with the indentations I being
oriented in directions of the second nearest neighbouring
columns.
[0070] A column of the third group is disposed inside the array,
i.e. is surrounded by neighbouring columns in all four directions.
As further illustrated in FIG. 4, an electrode element 210X inside
a regular, rectangular array 250 is surrounded by a first sphere
1SP of nearest neighbouring columns, the first sphere being marked
by a dashed line running through centres of the four nearest
electrode elements 210-1SP around electrode element 210X, which are
indicated by a hatch pattern. Electrode elements 210-2SP disposed
second nearest to electrode element 210X are indicated by a square
pattern, the second sphere 2SP being further indicated by dotted
line 2SP running through centres of the second nearest electrode
elements 210-2SP. A third sphere is further indicated by dashed
line 3SP. Further spheres could be marked in FIG. 6, with each
further sphere running through centres of respective electrode
elements disposed on a predetermined radius from the centre of the
aperture 210X, but are not indicated for sake of simplicity. As
apparent from FIG. 4, the arrangement of columns around column 210X
is highly symmetric and has fourfold symmetry. Therefore, a shape
of the aperture 213 of the electrode element 210 (210X) of the
third group has four indentations I from an ideal spherical shape,
the indentations being oriented in a direction of the second
nearest neighbours (210-2SP). This shape of aperture is
advantageously multi-pole corrected to compensate for electrostatic
fields generated by the symmetrical layout of neighbouring columns
as indicated in FIGS. 3 and 4. The description of shapes, as given
above, represents a simplified approach. Evidently, in other
embodiments, further shape features may be superimposed to the
shapes as described above, to take an influence of columns disposed
at a greater distance also into account.
[0071] In FIG. 5, the characteristics used herein to describe the
shape of the non-circular aperture 213 are illustrated. The
aperture 213 is an aperture 213 of an electrode element 210 of a
column of the third group of columns, as described above. The
non-circular shape, in particular a deviation of the shape of the
aperture 213 from an ideal circle is characterised herein by a
difference between a distance D1 from the centre of the aperture
213, which typically coincides with the optical axis, and a point
on the inner peripheral edge 212 forming the aperture 213, which
point is disclosed closest to the optical axis, i.e. is a shortest
distance of any point on the inner peripheral edge 212 from the
centre of the aperture 213. The centre of the aperture is marked by
a black dot. A second distance D2 is defined as the distance of a
point on the inner peripheral edge 212 that is disposed farthest
from the centre of the aperture 213. Thus, generally, the point
defining the first distance would be disposed on an indentation I.
The indentation I is a shape feature describing a portion of the
aperture, which deviates from an ideal spherical shape and extends
towards the centre of the aperture 213. An ideal spherical shape IC
may be defined to be a circle with a centre coinciding with the
centre of the aperture and having a radius equal to the second
distance D2. This definition was chosen for sake of simplicity. As
will be readily apparent to the skilled person, other definitions
may have been chosen to describe a non-circular shape. Inner
peripheral edge of outer ring member 220 is also depicted, with the
same considerations as given in connection with FIG. 3
applying.
[0072] In FIG. 6, a cut through array 250 along line A . . . A as
illustrated in FIGS. 3 and 4 is depicted. Array 250 is one of a
plurality of arrays 250A through 250D which are arranged in series
along the optical axes OA (a through e) of the individual columns a
through e and parallel to one another in a plane orthogonal to the
optical axes OAa through OAe. Each column a, b, c, d, e has an
electrostatic lens arrangement with a plurality of electrode
elements 210a, 210b, 210c, 210d, 210e, respectively, disposed
coaxially one after another in a stack along a respective optical
axis OAa, OAb, OAc, OAd, OAe of the column a, b, c, d, e. Each
electrode element 210 a through e comprises an inner electrode
member 211 around the respective optical axis OA. Each electrode
element 210 a through e further comprises a shielding member 222 of
cylindrical shape disposed in a region of an outer periphery of
inner electrode member 211 and on plate 230. Shielding member is
disposed in a gap between two adjacent arrays 250A/, B/C, C/D, D/E
in order to prevent electrostatic fields from penetrating inside
the respective electrostatic lens formed by respective electrode
elements. Plate 230 may be a mounting structure in the form of a
plate comprising apertures for accommodating inner electrode
members. For instance, inner electrode member 211 and shielding
member 222 may be attached to plate 230 independently, with the
inner electrode member 211 being inserted into a respective
aperture of the mounting plate whereas cylindrical shielding member
is disposed around a circumference of the respective aperture. In
alternative embodiments, plate 230 may be a portion of the
electrode element 210 comprising outer ring portion 220 as well as
connector 221 as depicted in FIG. 2, with respective portions 230
of neighbouring electrode elements 210 a through e (in the same
plane orthogonal to the optical axis) being connected to one
another. The electrode elements 210 and plate 230 may be integrally
formed, for instance. FIG. 6 illustrates the coaxial alignment of
the electrode members 211 (a through e, respectively) disposed in
series along the optical axis and configuration of electrode
elements 210 of neighbouring columns on a common mounting
structure. A few insulating spacer elements 280 arranged between
mounting plates 250A/B and B/C to space them apart are also
illustrated as an example. Of course, spacer elements could also be
disposed between the other mounting structures (not depicted for
ease of illustration). In addition, fewer spacer elements could be
used in between two given mounting plates, or they could be
disposed in different positions relative to the apertures and the
like, as will be readily apparent to the skilled person.
[0073] While the invention has been described also with respect to
certain specific embodiments thereof, it is evident that many
alternatives, modifications and variations will be apparent to
those skilled in the art. Accordingly, the preferred embodiments of
the invention set forth herein are intended to be illustrative and
not limiting in any way. Various changes may be made without
departing from the spirit and scope of the present invention as
defined in the following claims.
* * * * *